1. Field of the invention
This invention relates to a semiconductor laser apparatus producing a high output power, which can be used as a coherent light source in an optical communication system, an optical measuring system, an optical information processing system, etc.
2. Description of the prior art
In recent years, with the enlarged use of semiconductor lasers in fields such as optical communication, optical measurement, optical information processing, etc., semiconductor lasers producing laser light in a stabilized single longitudinal mode are required as a coherent light source.
For that reason, conventional semiconductor laser devices have been designed so that the oscillation longitudinal mode of the laser devices can be selected depending upon the gain distribution of laser media and the transmission characteristics of the laser resonator. FIGS. 7(a) to 7(c) show the selectivity of the oscillation longitudinal mode of a conventional semiconductor laser device, wherein FIG. 7(a) shows the relationship between the wavelength and the gain distribution of laser media, FIG. 7(b) shows the relationship between the wavelength and the spectrum of each longitudinal mode, and FIG. 7(c) shows the spectrum in the superradiant situation that is obtained by the superposition of the characteristic curves of FIGS. 7(a) and 7(b). In general, of the longitudinal modes of laser light produced by a semiconductor laser device, a longitudinal mode of laser light with a wavelength that is the closest to the peak (the maximum value) of a gain distribution receives the greatest gain and becomes an oscillation longitudinal mode. However, the bandgap of the semiconductor materials of the laser device varies with changes in the circumferential temperature, and accordingly the wavelength peak of the gain distribution is shifted to the long wavelength side at the rate of 2-3 .ANG./deg. Moreover, not only the refractive index of laser media varies, but the effective optical length of the laser cavity varies with changes in the thermal expansion of the laser device, so that the said longitudinal modes are shifted to the long wavelength side at the rate of about 0.7 .ANG./deg., while maintaining a distance of about 3 .ANG. therebetween. Thus, when the circumferential temperature rises, since the amount of change in the gain distribution is greater than that of change in the longitudinal modes at the beginning, the oscillation wavelength continuously varies over a period of time. When the temperature further rises, the oscillation wavelength brings about mode hopping, after which the oscillation wavelength causes the above process to repeat itself resulting in continuous changes and further mode hopping, as shown in FIG. 9, thereby attaining a step function type change. Moreover, the wavelength also varies with changes in current for driving the laser device. The above-mentioned phenomena prevent the application of conventional semiconductor laser devices as a light source for either wavelength multiplex optical-communication or a high resolution spectrometer.
In order to solve these problems, an SEC laser apparatus (short external cavity laser diode) has been invented in which laser light from the light-emitting rear facet of a semiconductor laser device is reflected by an external cavity and returns to the said semiconductor laser device. The oscillation longitudinal mode of the said laser device of the SEC laser apparatus is selected depending upon three factors, namely, the gain distribution of the laser, the longitudinal modes, and the selectivity of wavelength of the external cavity, which are shown in FIGS. 8(a), 8(b) and 8(d) in contradistinction to those of FIGS. 7(a) to 7(c). FIG. 8(a) shows the relationship between the wavelength and the gain distribution of laser media, FIG. 8(b) shows the relationship between the wavelength and the spectrum of each longitudinal mode, FIG. 8(c) shows the relationship between the wavelength and the resonance characteristics of the external cavity, and FIG. 8(d) shows the spectrum in the superradiant situation that is obtained by the superposition of the characteristic curves of FIGS. 8(a) to 8(c). The envelope curve of the spectrum in the superradiant situation of FIG. 8(d) is of a ripple, whereas that of the spectrum in the superradiant situation of FIG. 7(c) is of a smooth circular arch. The temperature characteristics of the peak of the envelope curve can be controlled by changes in the external cavity length (i.e., the distance between the semiconductor laser device and the external cavity), thereby attaining the suppression of mode hopping.
The characteristic of an oscillation wavelength with respect to temperatures of an ordinary SEC laser apparatus are shown in FIGS. 10(a) to 10(c), indicating that the same longitudinal mode is maintained in a temperature range of .DELTA.t; that a longitudinal mode successively receives the greatest gain in the same mountain of the envelope curve of the spectrum shown in FIG. 8(d) in a temperature range of .DELTA.T and becomes an oscillation longitudinal mode; and that when the temperature range exceeds .DELTA.T, the oscillation longitudinal mode is shifted to the adjacent mountain of the envelope curve, resulting in significant and sudden change of the oscillation longitudinal wavelength (i.e., mode hopping).
Moreover, FIG. 10(a) indicates that the temperature coefficient of the wavelength of the peak of the envelope curve shown in FIG. 8(d), d.lambda./dT, and the temperature coefficient of the longitudinal modes shown in FIG. 8(b), .alpha., have the following relationships: d.lambda./dT&lt;.alpha., the oscillation longitudinal mode is successively shifted to a longitudinal mode that is positioned at the short wavelength side a small mode hopping in a temperature range of .DELTA.T results. FIG. 10(b) indicates that when d.lambda./dT=.alpha., the value of .DELTA.T becomes equal to .DELTA.t, and the mode hopping does not occur in the range of .DELTA.T. FIG. 10(c) indicates that when d.lambda./dT&gt;.alpha., the oscillation longitudinal mode is successively shifted to a longitudinal mode that is adjacent to the long wavelength side, resulting in a small mode hopping in the range of .DELTA.T.
The above-mentioned SEC laser is composed of a VSIS (V-channelled Substrate Inner Stripe) semiconductor laser device with an active layer of AlGaAs grown on a GaAs substrate, and an external cavity with a high reflectivity coated by a multilayered dielectric film. Both facets of the laser device are coated by dielectric protective films with a thickness of half of the laser oscillation wavelength, thereby attaining a reflectivity of each of the facets, 0.32, with respect to the laser oscillation wavelength. When conventional SEC laser apparatuses are made with the above-mentioned combination of reflectivities, the level of an optical output power at which a stabilized laser operation can be achieved is low, about 10 mW at best. If the SEC laser apparatuses are operated with an optical output power at a higher level than that mentioned above, a kink occurs to the current-light characteristics of the SEC laser apparatuses because of turbulence of a transverse mode, which causes difficulties in maintaining the fundamental transverse mode and which makes the oscillation longitudinal mode unstable due to the occurrence of multi-mode oscillation, mode competitions, etc. The amount of current for driving the laser apparatuses must be increased with an increase in an optical output, which causes the laser apparatuses to be heated. As a result, the laser apparatuses give rise to an aging and/or deterioration of the characteristics, which causes difficulties in an achievement of high reliability.
Nevertheless, in recent years, laser apparatuses that oscillate laser light with a stabilized wavelength at high optical output power are required for use as a signal light source in optical information processing systems so as to heighten the density of signals, to shorten the period of time for treating the signals and to improve the signal to noise ratio.